DS2 did this. He had torticollis. Often times babies with this condition will end up in a helmet but we avoided it by doing early intervention. We did chiropractic care, OFTEN. There was a time when he went 4 days a week. They advised that I not put him in swings/bouncers and I didn't. I ended up carrying him in a Beco all the time and making sure I turned his head towards his less favored side until he could turn it himself. It was so much work especially because I had a wild child toddler at the time but was eventually fully resolved. He was a bit delayed with pushing up and crawling because of it.